Unbonded nonwoven web of polypropylene fibers



United States Patent 3,546,062 UNBONDED NONWOVEN WEB 0F POLYPROPYLENEFIBERS Arthur John Herrmau, Nashville, Tenn., assignor to E. I. du Pontde Nemours and Company, Wilmington, Del., a corporation of DelawareContinuation-impart of application Ser. No. 390,134, Aug. 17, 1964. Thisapplication Dec. 9, 1969, Ser. No. 883,652

Int. Cl. 133% /02; D04h 1/04 U.S. Cl. 161-169 4 Claims ABSTRACT OF THEDISCLOSURE Nonwoven webs of polypropylene fibers having low sensitivityto fluctuations in heat-bonding conditions are particularly useful asprecursors for carpet backings. To possess these qualities, the fibersof the web should have a crystallinity index of between about and 70, aco efiicient of variation in fiber-birefringence of at least 5% andgreater than 75% by weight of the fibers should have a birefringence ofat least 0.02.

DETAILED DESCRIPTION OF THE INVENTION This application is acontinuation-in-part of my application Ser. No. 390,134, filed Aug. 17,1964 now abandoned.

Bonded nonwoven sheets of like oriented polypropylene fibers have beendeveloped with a combination of properties which has made them of valuein such applications as primary and secondary backings for carpets,furniture foundation sheets, shoe fabrics, coated and laminatedstructures, filters, molded sheets and electrical and sound insulation.In particular, the combination of high tear strength and high tensilestrength, which is important in these applications, has been obtained byself-bonding the polypropylene fibers under carefully con trolledconditions of temperature, time of exposure to bonding conditions, andmaintenance of the Web under restraint while under bonding conditions.In the production of such sheets, careful temperature control of thebonding operation is required to avoid underbonding, which leads to lowtensile strength and overbonding which leads to low tear strength. Inaddition, the self-bonding operation must of necessity be carried outnear the softening point of the polypropylene fibers. Since heating thefibers at this temperature causes deorientation and an accompanyingdecrease in fiber tenacity, the bonding temperature must be preciselycontrolled to avoid an excessive loss in orientation. Loss inorientation is minimized by maintaining the web under restraint duringthe entire time it is at a temperature at which shrinkage of relaxedfibers will occur. By so minimizing loss in orientation and by carefultemperature control, it is possible to self-bond polypropylene fiberswhile confining the drop in fiber birefringence to less than andpreventing a drop in birefringence to a level below about 45% of themaximum birefringence (0.04 for polypropylene). In this way adequatefiber properties are retained in the bonded nonwoven sheet.

Even more precise temperature control is required in the self-bonding ofnonwoven webs of continuous polypropylene filaments which are to be usedas primary carpet backings; for not only must the bonded sheet have hightensile and tear strength, it is also necessary that the strength of theself-bonds be such that, during the tufting operation, the bonds breakrather than the filaments. It has been found that if the bonds breakduring tufting, the filaments have sufficient mobility to move under thestress of the tufting needles without excessive filament breakage.Excessive filament occurs with overbonded sheets and leads to tuftedcarpets which are deficient in grab-tensile strength.

If high grab-tensile strength were the only requirement of the tuftedcarpet, a lightly bonded sheet which could be easily prepared would besatisfactory. However, not only must the fibers be so bonded that theyare free to move with a minimum of fiber breakage during tufting, but asufficient number of bonds must also be retained after tufting to givethe sheet dimensional stability during subsequent processing. Forexample, with an inadequate number of self-bonds, the tufted nonwovensheet undergoes excessive shrinkage in the cross-machine directionduring passage through the dye beck. Thus the self-bonding must becarefully controlled to avoid both overbonding, which leads to excessivefiber-breakage and low tufted-grab-tensile strength, and underbonding,which gives high dye-beck-width losses. Temperature control within therange of 05 C. and preferably operation within a range of only 0.1 C. inall dimensions of the nonwoven web has been required for suchself-bonded sheets.

This critical temperature control and the maintenance of the requiredrestraint can be obtained by exposure of the web, while held between aporous belt and a solid belt, to a nonsolvating saturated atmosphere,for example steam, under pressure. By maintenance of a differentialsteam pressure on the two sides of the solid belt, an adequaterestraining force can be applied to the web to hold filament shrinkagewithin about 20%, and preferably within 5%. As mentioned above, thepreciseness of temperature control required is difficult to obtain, andis a disadvantage of these self-bonded nonwoven sheets. The problem isparticularly severe with wide sheets, for example those 15 ft. (4.6 m.)or more wide, which are used as primary carpet backings.

The purpose of this invention is to provide a nonwoven web ofpolypropylene fibers which has low sensitivity to bonding conditions.

Another purpose is to provide a nonwoven web of polypropylene fiberswhich can be bonded to a strong and tear-resistant sheet.

Still another purpose is to provide a nonwoven web which can be readilybonded in wide widths to form a nonwoven sheet having a good combinationof properties for use as a primary carpet backing.

These and other purposes of this invention are attained by providing anonwoven web comprising polypropylene fibers having a crystallinityindex of 30 to and having a distribution of orientation levels such thatthe coefiicient of variation of the fiber-birefringences is at least 5%with the proviso that less than 25% by weight of the fibers have abirefringence of under 0.02.

Coefficient of variation (CV) is used herein in accord with itscustomary meaning in statistical analysis and is defined as follows:

0v: it? 1 where AN is the arithmetic mean of the birefringence values,

An is the measured value of the birefringence of individual fibers, and

m is the number of measurements.

In order to obtain the required coefficient of variation, the nonwovenweb comprises either a combination of low-oriented fibers andhigh-oriented fibers derived from the same polymer, or fibers havingalternating highand low-oriented segments. In the case where acombination of low-oriented and high-oriented fibers are employed, theformer will be called binder fibers and the latter, matrix fibers. Whensuch a sheet is properly bonded, it will differ from the bonded sheetsof like polypropylene filaments, which contain only self-bonds betweenmatrix fibers, in having self-bonds between the binder fibers,self-bonds between the matrix fibers and interbonds between the matrixand binder fibers. This ability to form a variety of bonds is animportant characteristic of the nonwoven webs of this invention. Sincethe bonds can and do have different bond strengths, the bonded nonwovensheet has a wide distribution of bond strengths. It has been found thata wide distribution of bond strengths permits wide limits on the averagebond strength and bond concentration to give good products, thus lesscritical control of bonding conditions is required. The nonwoven webs ofthis invention may comprise staple fibers, continuous filaments orcombinations of the two. The nonwoven webs can be formed frompolypropylene fibers having the required coefficient of variation inbirefringence by the standard techniques known in the art formanufacturing nonwoven fabrics. As mentioned previously, the binderfibers can be incorporated into the web as separate low-oriented fibersor as the low-oriented segments of fibers having both highandlow-oriented segments along their length.

Preferred nonwoven webs are composed of continuous polypropylenefilaments which have a random distribution throughout the web and areseparate and independent of each other, except at filament crossoverpoints. They exhibit a low level of parallelism between filaments, thusindicating a relative freedom from aggregation or ropiness. Suchnonwoven structures have especially high tear and tensile strengthbecause of the continuous filaments and exhibit isotropic propertiesbecause of the random arrangement of the filaments. Structures of thistype can be made by the general procedure of British Pat. 932,482. Theprocess described in this patent involves an integrated spinning,orientation, and laydown of the filaments to give a random nonwoven webwhich is essentially free from filament aggregates. In this process, thefreshly spun filaments are electrostatically charged, forwarded toward aweb-laydown zone and then permitted to separate due to the appliedelectrostatic charge before web-laydown.

This process can be readily used to prepare nonwoven webs in which thebinder filaments are either separate low-oriented filaments or arelow-oriented segments of filaments having highand low-oriented segmentsalong their length. Separate binder and matrix filaments can be producedby using two separate spinning and drawing machines and combining thefilaments prior to or during web-laydown. They can also be produced bysplitting the threadline from the spinneret so that a part bypasses thedrawing operation. Another method involves the use of a spinneret withvarying capillary geometry which produces filaments with varyingresponses to the drawing operation. Nonwoven webs of filaments withsegments having different levels of orientation can be produced bypulsing the throughput of polymer going to the drawing operation, bypulsing the draw ratio in the drawing operation, or by variation of thedrawing temperature. This latter method can be carried out by passingthe filaments over a heated fluted feed roll in the drawing step. It hasbeen found that these various techniques can be used to produce nonwovenwebs of polypropylene filaments in which the distribution of orientation(as measured by filament-birefringence) among or along the filaments ischaracterized by a coefficient of variation of at least The orientedpolypropylene matrix fibers in the unbonded nonwoven web will normallyhave a birefringence in the range of 0.020 to 0.040, while that of thebinder fibers will normally be in the range of from less than 0.01 toabout 0.030 and will always be less than that of the matrix fibers. Lessthan and preferably no more than about 20% by weight of the fibers inthe web have a birefringence below 0.02.

In addition to the birefringence distrbution, another importantcharacteristic of the nonwoven webs of this invention is thecrystallinity in both the binder and matrix fibers. In order to obtainthe three types of bonds which are needed for good overall sheetproperties and low bonding sensitivity, it is required that the web canbe heated to a sufficiently high temperature to obtain some degree ofbonding between the matrix fibers without destroying the structure ofthe binder fibers. If the binder fibers were amorphous or had asignificantly different crystallinity index than the matrix fibers,their melting point would be much below that of the crystalline matrixfibers, and thus they could melt and lose their fibrous structure beforethe matrix fibers could self-bond.

The 5 to 10 C. differences in softening point between the matrix andbinder fibers of the web which can be obtained by having differences inlevels of orientation such that the coefficient of variation is at least5%, are ideally suited to obtaining the variety of bonds anddistribution of bond strengths which are needed for lowbondingsensitivity. Accordingly, it is desired that the crystallinity index ofthe matrix and binder fibers be at about the same level and within therange of to to minimize the effect of crystallinity on the softeningpoint differences between the matrix and binder fibers.

The present invention makes use of the discovery of an unusualrelationship to bonding sensitivity possessed by nonwoven webs ofpolypropylene fibers having a crystallinity index in the range of 30 to70. The finding that the bonding sensitivity of such webs unexpectedlydrops when the percent coefficient of variation in fiber-birefringenceis at least 5% permits the production of acceptable product over arelatively wide range of bonding conditions.

As indicated above, the bonding of the nonwoven webs of this inventionmust be carried out under controlled conditions to obtain the varioustypes of desired bonds; but compared with the production of self-bondednonwoven sheets of like polypropylene fibers, the bonding conditions aremuch less critical, and therefore, it is easier to produce a uniformproduct of acceptable properties. The aforementioned self-bondingtechnique as applied to polypropylene fibers of like orientationrequires a control of temperature within the range of about 0.5 C. andpreferably within 0.1 C. A change in bonding temperature greater thanthis amount would result in product with properties so different as tobe unacceptable in tensile, tear or tufted-grab-tensile or as wouldrequire different tufting or other processing conditions. However,bonding of webs having the required percent coefficient of variation infiber-birefringence can be carried out within a range of about 3 C. andpreferably within a range of 1 C. This wider latitude of operablebonding conditions is of great significance to the commercialmanufacture of bonded nonwoven sheets, particularly for the Wide widthsrequired in primary carpet backings.

Bonding of the webs of this invention (comprising fibers of varyingorientation) can be carried out in a manner similar to that describedpreviously for self-bonding webs in which the polypropylene fibers areall alike. Thus, exposure of the mixed-orientation web to a saturatedvapor atmosphere, for example steam, under sufficient pressure to givethe desired bonding temperature while maintaining the web underrestraint to minimize fiber shrinkage is a preferred method. The actualtemperature used will be lower than that required for self-bonding apolypropylene web in which the fibers have the same level of orientationas the matrix fibers of the mixed-orientation Web. This lowertemperature is advantageous since the matrix fibers under theseconditions show less or no tendency to undergo fiber deorientation. Inmany instances, the matrix fibers in mixed-orientation webs actuallyshow an increase in birefringence (greater orientation) after bonding,in contrast to the usual loss in fiber birefringence during self-bondingof polypropylene webs. The binder fibers in mixed-orientation webs losemuch of their orientation during bonding, thus the birefringence of thebinder fibers in typical mixed-orientation webs will drop from about0.02 to less than 0.001.

The temperature at which the nonwoven webs of this invention are bondeddepends on the rate the fibers are heated and the fiber crystallinity.While it is not intended to be limited by any particular theory ofoperation, it is believed that bonding occurs when the polymer as itexists in the fibers softens. A slow heat-up rate permits thecrystallinity to increase so that the softening point is raised andbonding can take place only at a higher temperature corresponding to thesoftening point of the higher crystalline form of the polymer. Thebonding temperature will approach the crystalline melting point of thebinder fibers in the mixed-orientation webs of this invention. In thecase of rapid heat-up rate, the bonding temperature is reached beforethe polymer in the fiber has an opportunity to increase substantially incrystallinity and hence a lower temperature can achieve bonding.Starting with the same nonwoven web of isotactic polypropylene fibers,the faster the heat-up rate, the lower will be the bonding temperaturenecessary to achieve a desired level or bonding.

Heat-up rate is dependent on both the bonding method and the apparatus.In the operation of a typical batchbonding process in an autoclave, thepressure of the saturated steam may be increased so that the heat-uprate of the web will be of the order of C. per minute. When operating atypical continuous process, the web is passed continuously through abonding chamber which is essentially closed to the atmosphere. Thechamber is pressurized with saturated steam at a constant pressure andunder these conditions the heat-up rate of the web may be of the orderof 250 C. per second. Thus, the heat-up rate, when operatingcontinuously may be as much as 500 times greater than when operatingbatch-wise. The bonding temperature employed in the continuing processcan be as much as 10 C. below that used in batchbonding.

Typical steam pressures used in the preferred continuous bonding of thenonwoven Webs of this invention, under conditions wherein the exposuretime of the web in the bonding chamber is about 6.2 seconds, are in therange of about 65 to 85 p.s.i.a. (4.6 to 6.0 kg./cm. which correspondsto a temperature of about 148 to 158 C. Using the same procedure with anonwoven web of polypropylene fibers, all of which have approximatelythe same level of orientation as the matrix fibers in themixed-orientation web, pressures of about 85 to 102 p.s.i.a. (6.0 to 7.2kg./cm. which correspond to temperatures of about 158 to 165 C., areused. Within these pressure ranges, the grab-tensile strength of atufted 4 oz./yd. (136 g./m. sheet varies by about 6 lb. per lb./in.change in bonding pressure (39 kg. per kg./cm. for the bonded sheet ofmixed-orientation polypropylene filaments and by about lb. per lb./in.change in bonding pressure (226 kg. per kg./cm. for the self-bondednonwoven sheet of uniformly oriented polypropylene filaments. Thisvariation is determined by measuring the slope of the curve oftuftedgrab-tensile strength versus bonding pressure at thetuftedgrab-tensile strength of 120 lb./( kg). This indicates that thenonwoven webs of this invention are much less sensitive to bondingconditions than the previously known polypropylene nowoven webs.

The lower bonding temperature and bonding sensitivity permits theproduction of useful products from these webs by calender bonding.Further evidence of this lower sensitivity of the nonwoven webs of thisinvention to bonding conditions is illustrated in the figure, in whichbonding sensitivity, expressed as the variation in strip tensilestrength with bonding pressure, is plotted against per cent coefficientof variation in fiber-birefringence. Bonding sensitivity in the figureis obtained by calculating the slope of the curve of strip tensilestrength versus bonding pressure within the range of 4 to 8 lbs./in.//oz./yd. (21 to 42 g./cm.//g./m. for the strip tensile strength ofnonwoven sheets of continuous polypropylene filaments. This range wasselected since it represents the optimum combination of tear and tensileproperties. It is seen that the bonding sensitivity rises rapidly as thecoefficient of variation falls below 5%. Above 5%, the bondingsensitivity decreases, that is, falls below 0.5 lb./in.// oz./yd./p.s.i.a. (37 g./cm.//g./m /kg./cm. providing that greater than 75% byweight of the fibers in the web have a birefringence that is at least0.02. It will be noted that where 25% or more by weight of the fibers inthe web have a birefringence of under 0.02 as in Examples 13-15, thebonding sensitivity rises substantially. This is evident from thedistance that the points representing Examples 13-15 are above the curvein the figure. With a typical nonwoven Web of polypropylene filamentshaving the same level of orientation, the bonding sensitivity is about0.8 lb./in.//oz./yd. /p.s.i.a. (60 g./cm.//g./m. kg./cm.

The polypropylene fibers used in the nonwoven webs of this invention areof textile denier, varying from about 1 to about 15 (0.1 to 1.7 tex).Higher deniers up to about 25 (2.8 tex) may be used for specialapplications. These filaments may be crimped or straight and may beround in cross section or have different shapes such as trilobal,elliptical, etc. The unbonded nonwoven webs may have a unit weight fromas low as about 0.5 oz./yd. (17 g./m. to 20 oz./yd. (680 g./m. orhigher, with webs having a unit weight of 2 to 5 oz./yd. (68 to 170g./m. being preferred as intermediates for preparation of primary carpetbackings.

Fiber-birefringence in the nonwoven webs of this invention can bemeasured by standard techniques known in the art. It is necessary todetermine the birefringence of an adequate number of the fibers for themeasurement to be representative of the entire nonwoven web. A measureof the crystallinity index of the polypropylene fibers can be obtainedby means of X-ray diffraction techniques.

The invention will be further illustrated by the following examples.

A series of nonwoven webs is prepared as described in Examples 1l6. Eachof the webs is then bonded continuously by passing the web at a speed of10 yd./min. (9 m./min.) while under restraint between one porous metalbelt and one solid belt, each faced with cloth, for a distance of 37inches (94 cm.) through a steam chamber in which saturated steam ismaintained at superatmospheric pressure. A differential steam pressureof 2 p.s.i. (0.14 kg./cm. is maintained on opposite sides of the solidbelt. The time of exposure of the web to saturated steam in the bondingchamber is about 6.2 seconds and the restraint on the web while in thechamber is 0.75 lb./in. //oz./yd. (1.56 g./cm. //g./m. The steamtemperature is varied to give a bonding pressure profile. The striptensile strength of the various bonded sheets is then determinedaccording to ASTM method D1117, except that a 5-in. (12.7 cm.) jawseparation is used With a strain rate on a 0.5-in. (1.27 cm.) widesample. The strip tensile strength is plotted against the pressure ofthe steam on the web side of the solid belt. The bonding sensitivity isobtained by calculating the slope of the curve of the strip tensileversus bonding within the range of 4 to 8 lb./in.//oz./yd. (21 to 42g./in.//g./m. The data on the bonded webs are summarized in Table l. Thebirefringence of the fibers in the unbonded nonwoven webs was determinedwith a Berek compensator known in the art. The crystallinity index isthe percentage ratio of the crystalline X-ray diffraction intensity tothe total diffraction intensity. Absolute values will vary according tothe specific details of the method and corrections used. For the purposeof this description and the following claims, values of birefringenceare determined using a Berek compensator and crystallinity indexdetermined by the method described below.

A Phillips (Norelco) diffractometer is employed for the crystallinityindex determination. The radiation is CuKu from a high intensitydiffraction tube operated at 40 milliamps and 40 kilovolts. The X-raybeam is defined by 4 divergence and scatter slits, a 0.006" receivingslit and a 4 take-off angle. A 0.001 Ni filter is used to eliminate CuKpradiation and the detector is a Nal(Tl) scintillation counter withpulse-height-analysis to minimize background from continuous radiation.The diffracted intensity is recorded in the usual manner with a ratemotor and strip-chart recorder.

The sample consists of a inch thick layer of the fibers to be examined.They are placed in the Philips sample spinner which is mounted such thatthe plane of rotation of the spinner bisects the incident and detectedX-ray beams. A mask having a inch diameter hole covers the sample. TheX-ray beam strikes the sample, while the sample is being rotated at 77rpm. The transmitted diffraction pattern is recorded between the anglesof 4 and 34 20, where equals the Bragg angle with scale factor settingssuch as to make the intensity of w the 110 peak near 14 about 70 to 80%full scale. The scanning rate is 1 20 per minute with a time constant of2 seconds. The chart speed is /2" per minute.

The diffractometer scan obtained by the above procedure is used in thefollowing manner in order to determine a crystallinity index. A baseline is drawn to inter- 8 peaks but bounded by the crystalline peaks, AThat is,

A =A +A A crystallinity index, C, is defined as:

C AT

In practice, it is more convenient to measure A and A in which case theequivalent expression is:

The fibers may be removed from the web prior to bonding for thedetermination of birefringence and crystallinity, or thesecharacteristics may be determined on fibers prior to the formation ofthe web. In either case, a representative sampling should be effected.

The relationship between bonding sensitivity and percent CV offilament-birefringence is shown in the figure. From the figure, it isseen that the use of webs having at least 5% CV in fiber-birefringenceis the key to obtaining bonded sheets of reproducible properties over arelatively wide range of bonding temperatures, such sheets having thecombination of high tear and tensile strength and preferably highgrab-tensile strength after tufting. Points 1-4, 10, 12 and 16representing the corresponding examples are illustrative of such webs.Points 1315 corresponding to Examples 13-15 represent webs with therequired percent CV, however, such webs have poor bonding sensitivitybecause 25% or more by weight of the fibers have a birefringence ofbelow 0.02. Examples 5-9 and 11 are unsuitable as evidenced by theirpoints on the figure falling below 5% CV.

TABLE 1 Lo\voriented filaments High-oriented filaments BirefringenceCrystal- Average Crystal- Average Bonding Tenacity linity birefrin-Tenacity linity birefrin- Web Percent sensig.p.d index gence g.p.d.index gence average V tivity X 4 03 0. 0288 0. 0208 17. 3 0. 40 4 30 370. 0203 O. 0277 14. 0 0. 37 4. 34 3.) 0. 0294 0. 0285 9. 0 0. 47 4. 1038 0. 0291 0. 0284 7. 7 0. 4. 00 40 0. 0287 0. 0287 4. 1 0. 63 4. 07 440. 0280 O. 0280 4. (i 0. "I. 00 47 0. 0201 0. 0291 3. 0 0. 00 3. 96 360. 0279 0. 0270 3. 1 0. 82 3. 82 42 0. 0280 0. 0282 4. 0 0. 71 4. 11 420. 0200 0. 0277 15. 5 0. 46 3. 75 40 0. 0276 0. 0270 3. 4 0. 70 4. 03 380. 0303 0. 0302 5. 0 0. 44 3. 86 43 0. 0330 0. 0280 10. 3 0. 55 4. 48 420. 0208 0. 0264 '13. 1 0. 56 3. 05 41 0. 030 0. 0252 24. 4 0. 73 5. 8334 0. 0330 0. 0317 20. 1 0. 25

Inlb./in.//oz./yd. /p.s.i.a. bonding pressure; to convert tog./em.//g./n1. /kg./cm. multiply by 75.

sect the scan at 7 and 32 26. This base line should appear tangent tothe scan at these angles. The intensity below this base line isconsidered background and is ignored. The intensity above this base lineis assumed to consist of coherent diffracted radiation from bothamorphous and crystalline regions. The following steps are employed toresolve the crystalline peaks from the underlying amorphous peak. First,a line is drawn tangent to the scan at both sides of the peak whichoccurs at about 14 26. The points of tangency will be near 12 and 16 20.This line is extended beyond the point of tangency near 16 and isterminated at 20. From this point, another line is drawn tangent to thescan on the high-angle side of the 131, 041, 111 triplet peak whichoccurs in the region 21 to 23 20. This point of tangency will be near 2420. Other tangents to the scan are drawn under the less intensecrystalline peaks located at 25 /2 and 20 20. This completes theresolution of the pattern. The amorphous peak defined by this procedureis a roughly triangular area with its apex at 165 20. It is defined inthe above manner because it is uniquely determined by the diffractionprofile.

The areas determined by the above procedures are measured with aplanimeter. The total area above the background line, A consists of thearea within the amorphous peak, A and the area above the amorphousEXAMPLE 1 A nonwoven web of 14% low-oriented and 86% highorientedcrystalline polypropylene filaments is prepared as follows: isotacticpolypropylene (melt flow rate (MFR) 12, by method of ASTM D-1238 at 230C. with a loading of 2.16 kg.) is spun through a 30-h0le spinneret at arate of 18 g./min. total and through a 5-hole spinneret at a rate of 3g./rnin. total. Each spinneret hole for both spinnerets is 0.015 in.(0.038 cm.) in diameter and the temperature of the 30-hole spinneret is242 C. and the 5-hole spinneret, 220 C. The filaments from the 30-holespinneret are led to a heated feed roll operating with a surfacetemperature of 118 C., and advanced by means of an idler roll cantedwith respect to the heated roll. A total of 5 wraps is used on theheated feed roll, which is operated with a surface speed of 243 yd./min.(222 m./min.). The filaments leaving the heated feed roll are thenpassed 5 Wraps around an idler roll/ draw roll system operating coldwith a surface speed of 858 yd./ min. (785 m./min.). These filaments aredrawn 3.5X, are 7.48 denier (0.83 tex) per filament and have a tenacityof 4.03 g.p.d. The filaments from the 5-hole spinneret are led to aheated roll operating with a surface temperature of 95 C. and a surfacespeed of 703 yd./min. (642 m./rnin.). The filaments are in contact withthe heated roll for 180. The filaments leaving the heated roll are thenpassed to a draw roll operating cold with a surface speed of 852yd./min. (779 m./min.). The filaments are in contact with the draw rollfor 180. These filaments are drawn 1.21X, are 7.73 denier (0.86 tex) perfilament and have a tenacity of 1.62 g.p.d. The filaments from bothspinnerets meet and are guided so the low-oriented fila- 'rnen ts aredispersed uniformly throughout the highoriented filaments. The filamentsare then electrostatically charged with a corona discharge device,passed into a draw jet and subsequently deposited on a moving belt toform a nonwoven web of randomly distributed continuous filaments.

When this web is bonded by the procedure described above at a saturatedsteam pressure of 75 p.s.i.a. (5.3 kg./cm. prepared for tufting byapplication of a polysiloxane lubricant; and then tufted under thefollowing conditions:

Gauge (distane between need1es)-0.188 in. (0.48'cm.)

Speed400 tufts/min; 7 tufts/in. (2.8 tufts/cm.)

Pile yarn-3700 denier (410 tex) continuous filament nylon Type pile-Loopa tufted carpet with a grab-tensile strength of 133 lb. (60 kg.) and adye-beck-width loss of 3.8% is obtained.

EXAMPLES 2-5 Four nonwoven webs of continuous polypropylene filamentsare prepared in the same general method as described in Example 1 exceptthat the draw ratioof the low-oriented filaments is varied. The surfacespeed of the draw roll used for the high-oriented filaments ismaintained at 858 yd./min. (785 m./min.), and that used for thelow-oriented filaments, at 852 yd./min. (779 m./

10 examples do not give the required percent CV for the nonwoven webs toqualify as products of this invention.

TABLE 3 Total throughp g./min.

27 110 z 9 0.018 in. X 0.250 in 18 inlet...

1(0.1046 cm. x 0.685 cm.)....

es. 0.0131 in. x 0.0655 in inlet-.. (0.0333 cm. x 0.166 cm.)

+ Spinneret hole dimensions as shown are diameter x length.

EXAMPLES 10-1 1 Two nonwoven webs of oriented polypropylene filamentshaving low-oriented segments along the filament length are prepared asfollows: polypropylene filaments are spun through a 30-hole spinneret ata rate of 18 g./min. total. Each spinneret hole is 0.015 in. (0.038 cm.)in diameter and the temperature of the spinneret is 234 C. The filamentsare led to a heated feed roll having a circumference of 18.75 in. (47.7cm.) with three grooves cut out 120 apart. The grooves are 1.25 in. (3.2cm.) wide in Example 10 and 0.25 in. (0.64 cm.)

min.). Identification of the variables is made in Table 2. as in Example11. The filaments are in contact with the TABLE 2 High-orientedfilaments Low-oriented filaments Draw Tenacity Draw Tenacity Exampleratio g.p.d. Denier Textile ratio g.p.d. Denier Textile EXAMPLE 6 rollfor 220 The surface temperature of the roll is A nonwoven web of 100%high-oriented polypropylene fibers is prepared as follows: polypropylenefilaments are spun through a SO-hole spinneret at a rate of 18 g./min.total. Each spinneret hole is 0.015 in. (0.038 cm.) in diameter and thetemperature of the spinneret is 234 C. The filaments are led to a heatedfeed roll operating with a surface temperature of 118 (3., and advancedby means of an idler roll canted with respect to the heated roll. Atotal of 5 wraps is used on the heated feed roll which is operated witha surface speed of 243 yd./min. (222 m./ min.). The filaments leavingthe heated feed roll are then passed 5 wraps around an idler roll/drawroll system operating cold with a surface speed of 858 yd./min. (785m./min.). The filaments are drawn 3.6X, are 7.2 denier (0.8 tex) perfilament and have a tenacity of 4.07 g.p.d. The drawn filaments are thenelectrostatically charged with a corona discharge device, passed into adraw jet, and subsequently deposited on a moving belt to form a nonwovenweb of randomly distributed continuous filaments.

EXAMPLES 7-9 Three nonwoven Webs of crystalline polypropylene filamentswere prepared in the same general manner as that described in Example 6,except the geometry of the holes in the spinneret is varied.Identification of the variables is made in Table 3. The results in Table1 indicate that the variations in spinneret geometry in these C. and isoperating with a surface speed of 243 yd./min. (222 m./min.). Thefilaments leaving the heated feed roll are then passed 3 Wraps around anidler roll/draw roll system operating cold with a surface speed of 858yd./min. (785 m./min.). As the filaments pass over the grooved feedroll, the portions of the filaments over a groove remain relatively cooland the portions of the filaments between the grooves come in contactwith the roll and are heated by conduction. The hot portions orient morethan the cold portions, thus the filaments consist of thick and thinsegments of low and high orientation. In Example 10, the high-orientedsegments are about 18 in. (46 cm.) long and have a denier of about 7(0.8 tex), and the low-oriented segments are about 2 in. (5 cm.) longand have a denier of about 15 (1.7 tex). The low-oriented segmentscomprise about 20% by weight of the filaments. After drawing, thefilaments are electrostatically charged with a corona discharge device,passed into a draw jet. and subsequently deposited on a moving belt toform a nonwoven web of randomly distributed continuous filaments.

EXAMPLES 12-15 Four nonwoven webs of continuous polypropylene filamentsare prepared in the same general manner as described in Example 1,except that the amount of loworiented polypropylene filaments is variedby changing the number of those filaments, keeping the number of 11high-oriented filaments constant. Identification of the variables ismade in Table 4.

1 Spinneret holes are 0.015 in. (0.038 cm.) in diameter x 0.075 in.(0.100

m.) long.

1 Spinneret holes are 0.020 in. (0.051 cm.) in diameter x 0.080 1n.(0.203 cm.) long.

EXAMPLE 16 A web is prepared as in Example 1 except that the feed rollspeed for the high orientation filaments (from the 30-hole spinneret) is155 yd./min. (142 m./min.), and the draw roll speed is 839 yd./min. (767m./min.). The low-orientation filaments (-hole spinneret) are made witha feed roll speed of 667 yd./min. (610 m./min.) and a draw roll speed of852 yd./min. (779 -rn./min.). Properties of the fibers and the web arepresented in Table 1.

EXAMPLE 17 TABLE 5 Matrix fiber Bonding pressure birefringence After WebP.s.1.a. Kg./em. As spun bonding Mixed-orientation 68 4. 8 0. 0312 0.0335 74 5. 2 0. 0312 0. 0326 78 5. 5 0. 0312 0. 0327 100% oriented 95 6.7 0.0317 0. 0280 97 6. 8 0. 0317 O. 0278 98 6. 9 0. 0317 0. 0255 What isclaimed is: 1. An unbonded nonwoven web of polypropylene fibers, saidfibers having a crystallinity index of between about 30 and and having acoefiicient of variation in fiber-birefringence among the fibers of atleast 5% and greater than by weight of said fibers having abirefringence of at least 0.02.

2. The nonwoven web of claim 1 wherein all the fibers have substantiallythe same level of crystallinity.

3. The nonwoven web of claim 1 wherein fibers are present havingsegments of varying orientation along their length thus providing therequired coefiicient of variation in fiber-birefringence.

' 4. An unbonded nonwoven web of polypropylene fibers, said fibershaving a crystallinity index of between about 30 and 70 and having acoefficient of variation in fiber-birefringence among the fibers of atleast 5% and no more than about 20% by weight of said fibers having abirefringence below 0.02.

References Cited UNITED STATES PATENTS 3,193,442 7/1965 Schulz et a1.l6ll69 3,396,071 8/1968 Couzens 161-170 US. Cl. X.R.

